Fluid catalytic cracking (FCC) is employed in petroleum refineries to convert high boiling hydrocarbon fractions of crude oil to more valuable products like Liquefied Petroleum Gas (LPG), gasoline and diesel. For this, heavy crude oil is chemically cracked into lighter hydrocarbon fractions having comparatively smaller chain of carbon atoms with the help of one or more catalysts. These high boiling hydrocarbons fractions are then introduced, in multiple streams, into a riser reactor section to undergo catalytic cracking. This results in lighter hydrocarbon fractions, which may be further sent to a fractional distillation column for extracting aforementioned valuable products.
In Fluid Catalytic Cracking (FCC), the atomization of hydrocarbon feed is very critical for contacting the hydrocarbon feed with catalyst particles. A uniform and narrow distribution of droplet size helps in faster vaporization of hydrocarbon feed leading to reduction in coke and better product selectivity.
In FCC, catalyst particles having particle size distribution in the range of 0-150 μm with average particle size of 70-90 μm are used to carry out the cracking reaction of hydrocarbon feed. Naphtha, which is a light hydrocarbon feed, normally has a boiling point up to 180 degree celsius. Heavy hydrocarbons such as vacuum residue normally boils over 370 degrees Celsius.
The feed is injected into the moving catalyst particles (said catalyst particles having temperature greater than 650 degree Celsius) from an apparatus for cracking in the form of droplets and the cracking of these feed molecules takes place in vapour phase on the active catalyst surface in a very short period of time. If the feed is injected without proper atomization, the contact of the feed droplets and catalyst particles will be poor and the heat transfer from the hot catalyst particle to feed will be less, resulting in low vaporisation of feed. Therefore, the hydrocarbon feed is required to be atomized into fine droplets which are of similar sizes of catalyst particles. This essentially helps to increase the contact of feed with the catalyst particles and the transfer of heat from the catalyst to feed for faster vaporization.
Uniform feed atomization will favour catalytic cracking, resulting in more desirable products and decrease in production of undesirable product (coke and dry gas). While designing an apparatus for catalytic cracking, the objective is to generate a narrow distribution of droplet size of hydrocarbon feed with sauter mean diameter (SMD) nearly equal to the average particle size of the catalyst particles. Bigger droplets will cause more penetration into the catalyst bed in riser and form coke and dry gas. Smaller droplet size will cause less penetration.
U.S. Pat. No. 6,142,457 describes a nozzle for atomization of a liquid stream. The nozzle comprises a primary conduit and a secondary conduit. The primary conduit is concentric to the secondary conduit and defines an annular space and a mixing zone. The hydrocarbon feed is introduced in the annular space between the primary conduit and the secondary conduit and the dispersion medium is introduced into the secondary conduit. In the mixing zone, the hydrocarbon stream is joined with the dispersion stream in a manner to force the hydrocarbon stream into the general shape of a thin film that surrounds the dispersion medium. The inner surface of the primary conduit within the mixing zone is a tapered surface that reduces the cross-sectional area of the primary conduit in the mixing zones to form the liquid film therein which is atomized as it exits the primary conduit outlet.
U.S. Pat. No. 6,902,707 describes a FCC feed injector wherein the atomizing medium is injected at multiple stages to decrease the feed droplet size. The feed injector comprises a plurality of inlets and plurality of mixing zones. The mixing zones are in fluid connection with each other. In one embodiment, the injector comprises an external sparger configured to define a first mixing zone. In another embodiment, the injector comprises a mixing tee configured to define the first mixing zone. The first mixing zone receives the first atomizing fluid and the hydrocarbon feed to form a first mixture. The second mixing zone receives a second atomizing fluid and the mixture from the first mixing zone to form a second mixture. The second mixture is, thereafter, dispensed into the riser reactor zone in a pre-determined spray pattern.
U.S. Pat. No. 5,794,857A describes a feed nozzle assembly for introducing steam and a heavy petroleum hydrocarbon into a reactor. The feed nozzle assembly comprises a hydrocarbon conduit and a diluent/dispersion conduit. The hydrocarbon conduit is concentric to the dispersion conduit to define an annular passage for introducing the hydrocarbon feed. The nozzle further comprises a first nozzle tip and a second nozzle tip. The first nozzle tip is attached to an outer end of the dispersion conduit. The first nozzle tip comprises two rows having a plurality of passageways therein for passage of the dispersion stream out of said dispersion conduit into said heavy petroleum hydrocarbon passing through said hydrocarbon conduit, thereby resulting in a mixture of steam and hydrocarbon. The second nozzle tip is attached to the hydrocarbon conduit for dispensing the mixture of steam and hydrocarbon out of said feed nozzle assembly.
As can be seen, apparatus disclosed in prior arts fall short in completely and efficiently atomizing a hydrocarbon feed. One reason for said inefficient atomization may be that, in most of the prior arts, the onset of mixing and breaking up of the hydrocarbon stream does not start at the initial length of the apparatus and therefore there is an improper mixing/atomization of the hydrocarbon feed with the diluent/dispersion stream at the final stages. Even if onset of atomization occurs at initial length of the apparatus, none of the available prior arts enable formation of a thin film of hydrocarbon at the initial length to provide an enhanced interface area of the hydrocarbon film for increasing the mixing of hydrocarbon with dispersion/diluent stream. Due to inefficient mixing at the initial length, the apparatus of prior arts have to incorporate number of mixing zones which unnecessarily complicates the designs of the apparatus. Inefficient atomization leads to non-uniformity in terms of diameter and velocity of the droplets of the atomized hydrocarbon feed. Moreover, it takes considerable time for such hydrocarbon feeds to vaporize. Delayed vaporization of the hydrocarbon feed in turn leads to slow and inadequate absorption of heat by the hydrocarbon droplets inside the riser reactor, thus leading to undesirable thermal cracking and excessive production of by-products such as coke.
Therefore, there is a constant need for simple and improved apparatus which can facilitate the onset of atomization at initial stages and generate droplets of hydrocarbon feed having SMD lesser than those available in the prior art, preferably in the range nearing the average particle size of catalyst particles.